Ionic liquids, electrolyte salts for electrical storage devices, liquid electrolytes for electrical storage devices, electrical double-layer capacitors, and secondary batteries

ABSTRACT

Electrical storage devices having excellent low-temperature properties can be obtained by using a quaternary salt (or ionic liquid) of general formula (1) below as an electrolyte salt for electrical storage devices or a liquid electrolyte for electrical storage devices. 
                         
In formula (1), R 1  to R 4  are each independently an alkyl group of 1 to 5 carbons or an alkoxyalkyl group of the formula R′—O—(CH 2 ) n —, with the proviso that at least one group from among R 1  to R 4  is the above alkoxyalkyl group. X is a nitrogen or phosphorus atom, and Y is a monovalent anion.

This application is a Divisional of co-pending application Ser. No.10/472,823 filed on Sep. 25, 2003 and for which priority is claimedunder 35 U.S.C. § 120. Application Ser. No. 10/472,823 is the nationalphase of PCT International Application No. PCT/JP02/02845 filed on Mar.25, 2002 under 35 U.S.C. § 371. The entire contents of each of theabove-identified applications are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to ionic liquids, electrolyte salts forelectrical storage devices, liquid electrolytes for electrical storagedevices, electrical double-layer capacitors, and secondary batteries.

BACKGROUND ART

An ionic compound generally forms crystals in which positively chargedcations and negatively charged anions pull electrostatically againsteach other. When this ionic compound is dissolved in various otherliquids, including water, it provides a liquid that carries electricity;that is, an electrolyte solution. Electrolyte solutions obtained bydissolving an ionic compound in an organic solvent are commonly used in,for example, nonaqueous electrolyte batteries and capacitors.

Some ionic compounds, when the temperature is raised, undergo activationof thermal motion to such an extent as to overcome the ionicinteractions, causing the compound itself to become liquid and capableof carrying electricity. A salt in such a state is generally referred toas a “molten salt.”

The chemical species present in the molten salt are all charged cationsor anions; no neutral atoms or molecules are present. Therefore,elements which cannot be obtained from an aqueous electrolyte solutionbecause they have too large a reducing or oxidizing power with respectto water, including metals such as alkali metals, aluminum andrare-earth elements, and non-metals such as fluorine, can beelectrolyzed in a molten salt and obtained in elemental form. This hasbecome a main industrial application of molten salts.

Some such molten salts maintain a liquid state at room temperature anddo not solidify even at very low temperatures. Such molten salts whichmaintain a liquid state at room temperature or lower are referred to inparticular as “room-temperature molten salts” or “ionic liquids.” Tominimize electrostatic interactions between the cations and anions whichmake up the ionic liquid, either or both are molecular ions of asubstantial size, and are moreover monovalent to minimize the charge andelectrostatic interactions.

Research is actively being pursued on applications for such ionicliquids in electrolytic deposition and in electrolytes for batteries andother purposes. However, because ionic liquids generally have a highmoisture absorption and are difficult to handle in air, suchapplications has yet to be fully realized.

In light of the above, the 1-ethyl-3-methylimidazolium tetrafluoroboratereported by Wilkes et al. in 1992 is a remarkable ionic liquid that canbe handled even in air. This new ionic liquid led to the synthesis ofmany ionic liquids which are combinations of numerous alkylimidazoliumcations having different side chains with various anions. Although theproperties and applications for these ionic liquids are being activelyinvestigated, there exists a desire for the development of various ionicliquids that can be more conveniently produced and are easy to handle.

Nonaqueous liquid electrolyte-type electrical double-layer capacitorscan be charged and discharged at a high current, and thus holdconsiderable promise as energy storage devices for such applications aselectrical cars and auxiliary power supplies.

Prior-art nonaqueous liquid electrolyte-type electrical double-layercapacitors are constructed of positive and negative polarizableelectrodes made primarily of a carbonaceous material such as activatedcarbon and a nonaqueous electrolyte solution. The composition of thenonaqueous electrolyte solution is known to have a large influence onthe withstand voltage and electrostatic capacitance of the capacitor.

The nonaqueous electrolyte solution is composed of an electrolyte saltand a nonaqueous organic solvent. Studies have been conducted on variouscombinations of such electrolyte salts and nonaqueous organic solvents.

For example, quaternary ammonium salts (e.g., JP-A 61-32509, JP-A63-173312, JP-A 10-55717) and quaternary phosphonium salts (e.g., JP-A62-252927) are commonly used as the electrolyte salt because of theirsolubility and degree of dissociation in organic solvents, as well astheir broad electrochemical stability range. Organic solvents that arecommonly used on account of their high dielectric constant, broadelectrochemical stability range and high boiling point include ethylenecarbonate, diethyl carbonate, propylene carbonate, butylene carbonate,γ-butyrolactone, acetonitrile and sulfolane.

However, in nonaqueous electrolyte-type electrical double-layercapacitors currently in use, the inadequate solubility of electrolytesalts (e.g., quaternary ammonium salts, quaternary phosphonium salts) inorganic solvents commonly used for this purpose limits the amount ofsalt that can be added, resulting in nonaqueous electrolyte solutions oflower ionic conductivity and electrical double-layer capacitors of lowerelectrostatic capacitance.

Moreover, because the electrolyte salts have a low solubility, they tendto crystallize at low temperatures, compromising the low-temperaturecharacteristics of the electrical double-layer capacitor.

In light of these circumstances, the objects of the invention are toprovide ionic liquids which can be easily and efficiently produced,electrolyte salts for electrical storage devices which have excellentsolubility in organic solvents for nonaqueous electrolyte solutions andhave a low melting point, liquid electrolytes for electrical storagedevices which include these electrolyte salts, and also electricaldouble-layer capacitors and secondary batteries of excellentlow-temperature properties which are constructed using such liquidelectrolytes.

We have conducted extensive investigations aimed at achieving the aboveobjects, as a result of which we have discovered that some quaternaryammonium salts and quaternary phosphonium salts bearing at least onealkoxyalkyl substituent have low melting points and excellentcharacteristics as ionic liquids.

Moreover, we have found that, because quaternary ammonium salts andquaternary phosphonium salts bearing at least one alkoxyalkylsubstituent have excellent solubility in nonaqueous organic solventsused in electrical storage devices and also have a low melting point,liquid electrolytes prepared using such quaternary salts can be obtainedto a higher concentration than previously possible and are less likelyto result in deposition of the electrolyte salt at low temperatures. Wehave also found that electrical double-layer capacitors manufacturedusing such liquid electrolytes have a high electrostatic capacitance andexcellent low-temperature characteristics.

Accordingly, the present invention provides the following.

-   (1) An ionic liquid characterized by having general formula (1)    below and a melting point of up to 50° C.

wherein R¹ to R⁴ are each independently an alkyl of 1 to 5 carbons or analkoxyalkyl of the formula R′—O—(CH₂)_(n)—, R′ being methyl or ethyl andthe letter n being an integer from 1 to 4, and any two from among R¹,R², R³ and R⁴ may together form a ring, with the proviso that at leastone of groups R¹ to R⁴ is an alkoxyalkyl of the above formula; X is anitrogen or phosphorus atom; and Y is a monovalent anion.

-   (2) The ionic liquid of (1) above which is characterized by having a    melting point of up to 25° C.-   (3) The ionic liquid of (1) or (2) above which is characterized in    that X is a nitrogen atom.-   (4) The ionic liquid of (3) above which is characterized in that X    is a nitrogen atom, R′ is methyl, and the letter n is 2.-   (5) The ionic liquid of (1) or (2) above which is characterized by    having general formula (2) below

wherein R′ is methyl or ethyl, X is a nitrogen or phosphorus atom, Y isa monovalent anion, Me signifies methyl and Et signifies ethyl.

-   (6) The ionic liquid of any one of (1) to (5) above which is    characterized in that Y is BF₄ ⁻, PF₆ ⁻, (CF₃SO₂)₂N⁻, CF₃SO₃ ⁻ or    CF₃CO₂ ⁻.-   (7) The ionic liquid of (5) above which is characterized by Having    general formula (3) below

wherein Me signifies methyl and Et signifies ethyl.

-   (8) An electrolyte salt for electrical storage devices, which salt    is characterized by being a quaternary salt of general formula (1)    below

wherein R¹ to R⁴ are each independently an alkyl of 1 to 5 carbons or analkoxyalkyl of the formula R′—O—(CH₂)_(n)—, R′ being methyl or ethyl andthe letter n being an integer from 1 to 4, and any two from among R¹,R², R³ and R⁴ may together form a ring, with the proviso that at leastone of groups R¹ to R⁴ is an alkoxyalkyl of the above formula; X is anitrogen or phosphorus atom; and Y is a monovalent anion.

-   (9) The electrolyte salt for electrical storage devices of (8) above    which is characterized by being a quaternary salt in which X is a    nitrogen atom.-   (10) The electrolyte salt for electrical storage devices of (9)    above which is characterized by being a quaternary salt in which X    is a nitrogen atom, R′ is methyl and the letter n is 2.-   (11) The electrolyte salt for electrical storage devices of (8)    above which is characterized by being a quaternary salt having    general formula (2) below

wherein R′ is methyl or ethyl, X is a nitrogen or phosphorus atom, Y isa monovalent anion, Me signifies methyl and Et signifies ethyl.

-   (12) The electrolyte salt for electrical storage devices of any one    of (8) to (11) above which is characterized in that Y is BF₄ ⁻, PF₆    ⁻, (CF₃SO₂)₂N⁻, CF₃SO₃ ⁻ or CF₃CO₂ ⁻.-   (13) The electrolyte salt for electrical storage devices of (11)    above which is characterized by having general formula (3) below

wherein Me signifies methyl and Et signifies ethyl.

-   (14) The electrolyte salt for electrical storage devices of any one    of (8) to (13) above which is characterized by having a melting    point of up to 25° C.-   (15) A liquid electrolyte for electrical storage devices which is    characterized by being composed solely of the ionic liquid of any    one of (1) to (7) above.-   (16) A liquid electrolyte for electrical storage devices which is    characterized by being composed solely of the electrolyte salt for    electrical storage devices of (14) above.-   (17) A liquid electrolyte for electrical storage devices which is    characterized by including at least one ionic liquid of any one    of (1) to (7) above and a nonaqueous organic solvent.-   (18) A liquid electrolyte for electrical storage devices which is    characterized by including at least one electrolyte salt for    electrical storage devices according to any one of (8) to (13) above    and a nonaqueous organic solvent.-   (19) The liquid electrolyte for electrical storage devices of (17)    or (18) above which is characterized in that the nonaqueous organic    solvent is a mixed solvent which includes as a main component    ethylene carbonate or propylene carbonate.-   (20) The liquid electrolyte for electrical storage devices of (17)    or (18) above which is characterized in that the nonaqueous organic    solvent is one selected from among ethylene carbonate, propylene    carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl    carbonate and diethyl carbonate, or a mixed solvent of two or more    thereof.-   (21) An electrical double-layer capacitor having a pair of    polarizable electrodes, a separator between the polarizable    electrodes and a liquid electrolyte, which electrical double-layer    capacitor is characterized in that the liquid electrolyte is a    liquid electrolyte for electrical storage devices according to any    one of (15) to (20) above.-   (22) The electrical double-layer capacitor of (21) above which is    characterized in that the polarizable electrodes include as a main    component a carbonaceous material prepared from a resin.-   (23) The electrical double-layer capacitor of (22) above which is    characterized in that the resin is a phenolic resin or a    polycarbodiimide resin.-   (24) The electrical double-layer capacitor of (22) above which is    characterized in that the carbonaceous material is prepared by    carbonizing a phenolic resin or polycarbodiimide resin, then    activating the carbonized resin.-   (25) An electrolyte solution for electrical storage devices which is    characterized by being composed of the liquid electrolyte for    electrical storage devices of (15) or (16) above and an    ion-conductive salt which is solid at ambient temperature.-   (26) The electrolyte solution for electrical storage devices of (25)    above which is characterized in that the ion-conductive salt is a    lithium salt.-   (27) The electrolyte solution for electrical storage devices of (25)    or (26) above which is characterized by including also a nonaqueous    organic solvent.-   (28) A secondary battery having a positive electrode and a negative    electrode, a separator between the positive and negative electrodes,    and an electrolyte solution, which secondary battery is    characterized in that the electrolyte solution is an electrolyte    solution for electrical storage devices according to any one of (25)    to (27) above.-   (29) An electrical double-layer capacitor having a pair of    polarizable electrodes, a separator between the polarizable    electrodes and a liquid electrolyte, which electrical double-layer    capacitor is characterized in that the liquid electrolyte is an    electrolyte solution for electrical storage devices according to any    one of (25) to (27) above.-   (30) An electrical storage device having a positive electrode and a    negative electrode, a separator between the positive and negative    electrodes, and a liquid electrolyte, which electrical storage    device is characterized in that the positive electrode is activated    carbon, the negative electrode is a carbonaceous material that is    capable of occluding and releasing lithium ions, and the liquid    electrolyte is an electrolyte solution for electrical storage    devices according to any one of (25) to (27) above.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a chart showing the NMR spectrum for compound (3).

FIG. 2 is a chart showing the NMR spectrum for compound (4).

FIG. 3 is a chart showing the NMR spectrum for compound (5).

FIG. 4 is a chart showing the NMR spectrum for compound (6).

FIG. 5 is a chart showing the NMR spectrum for compound (8).

FIG. 6 is a chart showing the NMR spectrum for compound (9).

FIG. 7 is a chart showing the NMR spectrum for compound (10).

FIG. 8 is a chart showing the NMR spectrum for compound (11).

FIG. 9 is a graph of discharge capacity (room temperature) versusinitial charge/discharge cycles in the electrical double-layercapacitors obtained in Example 8 according to the invention andComparative Example 3.

FIG. 10 is a graph showing the temperature dependence of the dischargeperformance in the electrical double-layer capacitors obtained inExample 8 according to the invention and Comparative Example 3.

FIG. 11 is a graph showing the change over time in voltage (roomtemperature) after initial charging of the electrical double-layercapacitors obtained in Example 8 according to the invention andComparative Example 3.

FIG. 12 is a graph showing the charge and discharge characteristics ofthe secondary battery obtained in Example 9 according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention is described more fully below.

Electrolyte Salt for Electrical Storage Devices

The inventive electrolyte salts for electrical storage devices arequaternary salts of general formula (1) below

In the formula, R¹ to R⁴ are each independently an alkyl of 1 to 5carbons or an alkoxyalkyl of the formula R′—O—(CH₂)_(n)—, wherein R′ ismethyl or ethyl and the letter n is an integer from 1 to 4. Any two fromamong R¹, R², R³ and R⁴ may together form a ring. At least one of groupsR¹ to R⁴ is an alkoxyalkyl of the above formula. X is a nitrogen orphosphorus atom, and Y is a monovalent anion.

“Electrical storage device,” as used in the invention, refers to adevice or element which chemically, physically or physicochemicallystores electricity. Illustrative examples include devices capable ofbeing charged and discharged, such as capacitors—including electricaldouble-layer capacitors, and secondary batteries.

Exemplary alkyls having 1 to 5 carbons include methyl, ethyl, propyl,2-propyl, butyl and pentyl. Exemplary alkoxyalkyl groups of the formulaR′—O—(CH₂)_(n)— include methoxymethyl, ethoxymethyl, methoxyethyl,ethoxyethyl, methoxypropyl, ethoxypropyl, methoxybutyl and ethoxybutyl.

Exemplary compounds in which two groups from among R¹, R², R³ and R⁴together form a ring include, when X is a nitrogen atom, quaternaryammonium salts containing an aziridine, azetidine, pyrrolidine orpiperidine ring; and, when X is a phosphorus atom, quaternaryphosphonium salts containing a pentamethylenephosphine (phosphorinane)ring.

Quaternary ammonium salts having as a substituent at least onemethoxyethyl group in which R′ above is methyl and the letter n is 2 arepreferred.

Preferred use can also be made of quaternary salts of general formula(2) below having as substituents a methyl group, two ethyl groups and analkoxyethyl group.

In formula (2), R′ is methyl or ethyl, X is a nitrogen or phosphorusatom, and Y is a monovalent anion. In addition, Me represents a methylgroup and Et represents an ethyl group.

No particular limitation is imposed on the monovalent anion Y in generalformulas (1) and (2). Illustrative examples include BF₄ ⁻, PF₆ ⁻, AsF₆⁻, SbF₆ ⁻, AlCl₄ ⁻, NbF₆ ⁻, HSO₄ ⁻, ClO₄ ⁻, CH₃SO₃ ⁻, CF₃SO₃ ⁻, CF₃CO₂⁻, (CF₃SO₂)₂N⁻, Cl⁻, Br⁻ and I⁻. From the standpoint of such propertiesas the degree of dissociation, stability and ion mobility in thenonaqueous organic solvent, the use of BF₄ ⁻, PF₆ ⁻, (CF₃SO₂)₂N⁻, CF₃SO₃⁻ or CF₃CO₂ ⁻ is especially preferred.

Of the quaternary salts of above general formulas (1) and (2), specificexamples of quaternary ammonium salts and quaternary phosphonium saltspreferred for use in the practice of the invention include compounds (3)to (11) below (wherein Me represents methyl and Et represents ethyl).The quaternary ammonium salts of formulas (3) and (8) below areespecially preferred because they enable electrical storage deviceshaving excellent low-temperature characteristics to be obtained.

A common method for synthesizing the above quaternary ammonium salts isdescribed. First, a tertiary amine is mixed with a compound such as analkyl halide or a dialkyl sulfate. If necessary, the mixture is heated,giving a quaternary ammonium halide. Reaction under applied pressure,such as in an autoclave, is preferred when using a compound having lowreactivity, such as an alkoxyethyl halide or an alkoxymethyl halide.

The resulting quaternary ammonium halide is dissolved in an aqueoussolvent such as water and reacted with a reagent that generates therequired anionic species, such as tetrafluoroboric acid ortetrafluorophosphoric acid, to effect an anion exchange reaction,thereby yielding the quaternary ammonium salt of the invention.

In one illustrative method for synthesizing quaternary ammoniumtetrafluoroborates, a quaternary ammonium halide is dissolved in water,silver oxide is added and a salt exchange reaction is carried out toform the corresponding quaternary ammonium hydroxide. The product isthen reacted with tetrafluoroboric acid, yielding the target compound.This method is effective for synthesizing high-purity quaternaryammonium tetrafluoroborates because the silver halide that forms as aresult of salt exchange during formation of the quaternary ammoniumhydroxide can easily be removed.

Quaternary phosphonium salts can generally be synthesized in much thesame way as quaternary ammonium salts. Typically, a tertiary phosphineis mixed with a suitable compound such as an alkyl halide or a dialkylsulfate. If necessary, the reaction is carried out under the applicationof heat.

As in the case of quaternary ammonium salts, quaternary phosphoniumsalts containing various different anions may be prepared by dissolvinga quaternary phosphonium halide (a chloride, bromide or iodide) in anaqueous solvent and reacting the dissolved halide with a reagent thatgenerates the required anionic species so as to effect an anion exchangereaction.

To discourage deposition of the electrolyte salt when an electrolytesolution of the salt dissolved in a nonaqueous organic solvent is placedunder low-temperature conditions, it is preferable for the electrolytesalt to have a melting point not higher than 25° C., and preferably nothigher than 15° C. An electrolyte salt having a melting point higherthan 25° C. deposits out of the solvent at low temperatures, and is thusmore likely to lower the ionic conductivity of the electrolyte solutionand in turn reduce the amount of electricity that can be drawn from theelectrical storage device. The melting point is not subject to any lowerlimit, although a lower melting point is better.

Ionic Liquid

The ionic liquid according to the present invention is characterized byhaving general formula (1) below and a melting point of up to 50° C.,and preferably up to 25° C.

In the formula, R¹ to R⁴ are each independently an alkyl of 1 to 5carbons or an alkoxyalkyl of the formula R′—O—(CH₂)_(n)— (R′ beingmethyl or ethyl and the letter n being an integer from 1 to 4) and anytwo from among R¹, R², R³ and R⁴ may together form a ring, with theproviso that at least one of groups R¹ to R⁴ is an alkoxyalkyl of theabove formula. X is a nitrogen or phosphorus atom, and Y is a monovalentanion.

Compounds in which two groups from among the alkyls of 1 to 5 carbonsR¹, R², R³ and R⁴ together form a ring are exemplified by the samecompounds as mentioned above for electrolyte salts.

In this ionic liquid as well, quaternary ammonium salts having as asubstituent at least one methoxymethyl group in which R′ above is methyland the letter n is 2 are preferred.

Preferred use can also be made of quaternary salts of general formula(2) below having as substituents a methyl group, two ethyl groups and analkoxyethyl group.

In the formula, R′ is methyl or ethyl, X is a nitrogen or phosphorusatom, and Y is a monovalent anion. In addition, Me represents a methylgroup and Et represents an ethyl group.

The monovalent anion Y in the ionic liquid of above general formulas (1)and (2) is exemplified by the same monovalent anions as mentioned abovefor electrolyte salts.

Specific examples of ionic liquids include compounds of above formulas(3) to (11). The ionic liquids of formulas (3) and (8) are especiallypreferred because they are easy to handle and they enable electricalstorage devices having excellent low-temperature characteristics to beobtained.

The ionic liquid may be prepared in the same way as described above forthe electrolyte salt.

The ionic liquids of the invention have numerous desirable features.That is, they (1) have a vapor pressure that is either non-existent orvery low, (2) are non-flammable or flame-retarding, (3) have ionicconductivity, (4) have a higher decomposition voltage than water, (5)have a broader liquid temperature range than water, (6) can be handledin air, and (7) have a broader potential window than organic ionicliquids known to the prior art. In particular, when an ionic liquid isused in an electrical storage device, if the potential window is narrow,the electrolyte or electrolyte solution may undergo oxidativedecomposition or reductive decomposition. Imidazolium-type ionic liquidshave a narrow potential window, and so cannot be used in lithium ionsecondary battery systems. However, as noted above, the ionic liquids ofthis invention have a broad potential window, enabling them to be usedin lithium ion secondary batteries as well.

Accordingly, the inventive ionic liquids can be advantageously used asnovel electrolytes capable of functioning at temperatures below roomtemperature in the electrodeposition of metals and alloys, inelectroplating baths, and in electrochemical devices for storing energy,such as various types of batteries and capacitors.

Most reaction solvents that are widely used in organic synthesis, suchas benzene, methylene chloride and ether, are volatile substances havingcarcinogenicity. Yet, the ionic liquids of this invention have very lowvolatilities and also lend themselves well to use as repeatedly reusablereaction solvents for organic synthesis. Hence, they are capable ofcontributing also to the field of “green chemistry” which is developingnew synthetic processes that are less burdensome on the environment.

Liquid Electrolyte for Electrical Storage Devices

The inventive liquid electrolytes for electrical storage devices may beused in any of the following forms: (1) liquid electrolytes consistingsolely of the above-described ionic liquids or low-melting electrolytesalts for electrical storage devices (i.e., liquid electrolytes in whicha nonaqueous organic solvent is not used), (2) electrolyte solutionsobtained by adding an ion-conductive salt to above liquid electrolyte(1) (here too, a nonaqueous organic solvent is not used in the liquidelectrolyte), (3) electrolyte solutions obtained by adding also anonaqueous organic solvent to above electrolyte solution (2), and (4)electrolyte solutions containing at least one of the above-describedionic liquids or electrolytes for electrical storage devices incombination with a nonaqueous organic solvent.

Any nonaqueous organic solvent which is capable of dissolving theabove-described ionic liquid or electrolyte salt and is stable withinthe working voltage range for electrical storage devices such assecondary batteries and electrical double-layer capacitors may be usedwithout particular limitation. However, it is preferable for thenonaqueous organic solvent to be one having a large dielectric constant,a broad electrochemical stability range, a broad service temperaturerange and excellent safety.

Illustrative examples of suitable solvents include acyclic ethers suchas dibutyl ether, 1,2-dimethoxyethane, 1,2-ethoxymethoxyethane, methyldiglyme, methyl triglyme, methyl tetraglyme, ethyl glyme, ethyl diglyme,butyl diglyme, and glycol ethers (e.g., ethyl cellosolve, ethylcarbitol, butyl cellosolve, butyl carbitol); cyclic ethers such astetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane and4,4-dimethyl-1,3-dioxane; butyrolactones such as γ-butyrolactone,γ-valerolactone, δ-valerolactone, 3-methyl-1,3-oxazolidin-2-one and3-ethyl-1,3-oxazolidin-2-one; and solvents commonly used inelectrochemical devices, such as amide solvents (e.g.,N-methylformamide, N,N-dimethylformamide, N-methylacetamide,N-methylpyrrolidinone), carbonate solvents (e.g., diethyl carbonate,dimethyl carbonate, ethyl methyl carbonate, propylene carbonate,ethylene carbonate, styrene carbonate), and imidazolidinone solvents(e.g., 1,3-dimethyl-2-imidazolidinone). Any one or mixtures of two ormore of these solvents may be used.

The use of a mixed solvent which includes as a main component ethylenecarbonate or propylene carbonate, or of one or a mixture of two or moresolvents selected from among ethylene carbonate, propylene carbonate,vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate anddiethyl carbonate, is preferred.

When the above-described liquid electrolyte is used as a liquidelectrolyte for electrical storage devices, in the form described in (1)above it is of course 100% ionic liquid. In above forms (2), (3) and(4), the concentration of ionic liquid or electrolyte salt in thesolvent, while not subject to any particular limitation, is generally0.1 to 5.0 mol/L, and preferably 1.0 to 4.0 mol/L. At a concentration ofless than 0.1 mol/L, energy loss may rise due to increased internalresistance. On the other hand, at a concentration higher than 5.0 mol/L,if the electrolyte salt has a low solubility and a relatively highmelting point, undesirable effects may arise at low temperatures, suchas deposition of the salt and a decline in stability.

Because the electrolyte salts for electrical storage devices of theinvention have a better solubility in nonaqueous organic solvents thanconventional electrolyte salts and have a melting point no higher than25° C., the electrolyte salt does not readily deposit out of solution atlow temperatures even when used at a higher electrolyte saltconcentration than is normally the practice.

As noted above, an ion-conductive salt may be added to the liquidelectrolyte.

In this case, the ion-conductive salt may be any that is capable ofbeing used in electrical storage devices, such as lithium secondarycells, lithium ion secondary cells and electrical double-layercapacitors. Ion-conductive salts that may be used include alkali metalsalts and quaternary ammonium salts.

Preferred alkali metal salts are lithium salts, sodium salts andpotassium salts. Specific examples include: (1) lithium salts such aslithium tetrafluoroborate, lithium hexafluorophosphate, lithiumperchlorate, lithium trifluoromethanesulfonate, the sulfonyl imidelithium salts of general formula (12) below, the sulfonyl methidelithium salts of general formula (13) below, lithium acetate, lithiumtrifluoroacetate, lithium benzoate, lithium p-toluenesulfonate, lithiumnitrate, lithium bromide, lithium iodide and lithium tetraphenylborate;(2) sodium salts such as sodium perchlorate, sodium iodide, sodiumtetrafluoroborate, sodium hexafluorophosphate, sodiumtrifluoromethanesulfonate and sodium bromide; and (3) potassium saltssuch as potassium iodide, potassium tetrafluoroborate, potassiumhexafluorophosphate and potassium trifluoromethanesulfonate.(R^(a)—SO₂)(R^(b)—SO₂)NLi  (12)(R^(c)—SO₂)(R^(d)—SO₂)(R^(e)—SO₂)CLi  (13)

In above formulas (12) and (13), R^(a) to R^(e) are each independentlyC₁₋₄ perfluoroalkyl groups which may have one or two ether linkages.

Illustrative examples of the sulfonyl imide lithium salts of generalformula (12) include (CF₃SO₂)₂NLi, (C₂F₅SO₂)₂NLi, (C₃F₇SO₂)₂NLi,(C₄F₉SO₂)₂NLi, (CF₃SO₂)(C₂F₅SO₂)NLi, (CF₃SO₂)(C₃F₇SO₂)NLi,(CF₃SO₂)(C₄F₉SO₂)NLi, (C₂F₅SO₂)(C₃F₇SO₂)NLi, (C₂F₅SO₂)(C₄F₉SO₂)NLi and(CF₃OCF₂SO₂)₂NLi.

Illustrative examples of the sulfonyl methide lithium salts of generalformula (13) include (CF₃SO₂)₃CLi, (C₂F₅SO₂)₃CLi, (C₃F₇SO₂)₃CLi,(C₄F₉SO₂)₃CLi, (CF₃SO₂)₂(C₂F₅SO₂)CLi, (CF₃SO₂)₂(C₃F₇SO₂)CLi,(CF₃SO₂)₂(C₄F₉SO₂)CLi, (CF₃SO₂)(C₂F₅SO₂)₂CLi, (CF₃SO₂)(C₃F₇SO₂)₂CLi,(CF₃SO₂)(C₄F₉SO₂)₂CLi, (C₂F₅SO₂)₂(C₃F₇SO₂)CLi, (C₂F₅SO₂)₂(C₄F₉SO₂)CLiand (CF₃OCF₂SO₂)₃CLi.

Of the above, lithium tetrafluoroborate, lithium hexafluorophosphate,sulfonyl methide lithium salts of general formula (12) and generalformula (13) are preferred because they are ion-conductive salts havinga particularly high ionic conductivity and excellent thermal stability.These ion-conductive salts may be used singly or as combinations of twoor more thereof.

Quaternary ammonium salts that may be used in electrical double-layercapacitors include tetramethylammonium hexafluorophosphate,tetraethylammonium hexafluorophosphate, tetrapropylammoniumhexafluorophosphate, methyltriethylammonium hexafluorophosphate,tetraethylammonium tetrafluoroborate and tetraethylammonium perchlorate;and also acylic amidines, cyclic amidines (e.g., imidazoles,imidazolines, pyrimidines, 1,5-diazabicyclo[4.3.0]non-5-ene (DBN),1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)), pyrroles, pyrazoles,oxazoles, thiazoles, oxadiazoles, thiadiazoles, triazoles, pyridines,pyrazines, triazines, pyrrolidines, morpholines, piperidines andpiperazines.

The ion-conductive salt has a concentration in the electrolyte solutionof generally 0.05 to 3 mol/L, and preferably 0.1 to 2 mol/L. Too low anion-conductive salt concentration may make it impossible to obtain asufficient ionic conductivity, whereas too high a concentration mayprevent complete dissolution in the liquid electrolyte.

Electrical Double-Layer Capacitor

The electrical double-layer capacitor of the invention is composed of apair of polarizable electrodes, a separator between the polarizableelectrodes, and a liquid electrolyte, the latter being a liquidelectrolyte of the type described above for use in electrical storagedevices.

The polarizable electrodes may be ones produced by coating a currentcollector with a polarizable electrode composition containing acarbonaceous material and a binder polymer.

The carbonaceous material is not subject to any particular limitation.Illustrative examples include carbonaceous materials prepared by thecarbonization of a suitable starting material, or by both carbonizationand subsequent activation of the carbonized material to yield activatedcarbon. Examples of suitable starting materials include plant-basedmaterials such as wood, sawdust, coconut shells and pulp spent liquor;fossil fuel-based materials such as coal and petroleum fuel oil, as wellas fibers spun from coal or petroleum pitch obtained by the thermalcracking of such fossil fuel-based materials or from tar pitch; andsynthetic polymers, phenolic resins, furan resins, polyvinyl chlorideresins, polyvinylidene chloride resins, polyimide resins, polyamideresins, polycarbodiimide resins, liquid-crystal polymers, plastic wasteand reclaimed tire rubber.

Of the above, to prevent a decline in performance due to variability ofthe starting material or impurities in the starting material, it ispreferable for the carbonaceous material to be composed primarily of aresin-derived carbonaceous material. A carbonaceous material obtained bythe carbonization of a phenolic resin or polycarbodiimide resin,followed by activation is especially preferred.

Any known type of phenolic resin may be used without particularlimitation. Illustrative examples include resole-type resins, novolakresins, and other special phenolic resins.

Polycarbodiimide resins obtained by any of various known processes maylikewise be used without particular limitation (see U.S. Pat. No.2,941,966, JP-B 47-33279, J. Org. Chem. 20, 2069-2075 (1963), etc.). Forexample, use may be made of a polycarbodiimide resin prepared by thedecarboxylative condensation of an organic diisocyanate.

The method of activation is not subject to any particular limitation.Examples of such techniques that may be used include chemical activationand steam activation. Activated carbon prepared by chemical activationusing KOH is preferred because the resulting capacitor tends to have alarger electrostatic capacitance than when steam-activated carbon isused.

The carbonaceous material used in the practice of the invention may bein any of various forms, including shredded material, granulatedmaterial, pellets, fibers, felt, woven fabric or sheet.

A conductive material may be added to the carbonaceous material. Anyconductive material capable of imparting conductivity to thecarbonaceous material may be used without particular limitation.Illustrative examples include carbon black, Ketjenblack, acetyleneblack, carbon whiskers, carbon fibers, natural graphite, syntheticgraphite, titanium oxide, ruthenium oxide, and metallic fibers such asaluminum and nickel. Any one or combinations of two or more of the abovemay be used. The use of Ketjenblack, which is a type of carbon black, oracetylene black is preferred.

No particular limitation is imposed on the average particle size of theconductive material, although it is desirable for the conductivematerial to have an average particle size of preferably 10 nm to 10 μm,more preferably 10 to 100 nm, and most preferably 20 to 40 nm. Inparticular, it is advantageous for the conductive material to have anaverage particle size within a range of 1/5000 to ½, and especially1/1000 to 1/10, the average particle size of the carbonaceous material.

The amount of addition is not subject to any particular limitation.However, to achieve a good electrostatic capacitance and a goodconductivity imparting effect, addition in an amount of 0.1 to 20 partsby weight, and especially 0.5 to 10 parts by weight, per 100 parts byweight of the carbonaceous material is preferred.

The binder polymer may be any polymer suitable for use in the presentapplication. Preferred examples include (I) unsaturated polyurethanecompounds; (II) polymeric materials having an interpenetrating networkstructure or a semi-interpenetrating network structure; (III)thermoplastic resins containing units of general formula (14) below; and(IV) fluoropolymer materials. The use of any of polymeric materials (I)to (III) results in a high adhesion, and can therefore increase thephysical strength of the electrodes. As for fluoropolymer materials(IV), these have excellent thermal and electrical stability.

In the formula, the letter r is an integer from 3 to 5 and the letter sis an integer which is 5 or higher.

The above-described unsaturated polyurethane compounds (I) arepreferably ones prepared by reacting (A) an unsaturated alcohol havingat least one (meth)acryloyl group and a hydroxyl group on the molecule,(B) a polyol compound of general formula (15) below, (C) apolyisocyanate compound, and (D) an optional chain extender.HO—[(R⁵)_(h)-(Z)_(i)-(R⁶)_(j)]_(q)—OH  (15)In the formula, R⁵ and R⁶ are each independently a divalent hydrocarbongroup of 1 to 10 carbons which may contain an amino, nitro, carbonyl orether group; Z is —COO—, —OCOO—, —NR⁷CO— (R⁷ being a hydrogen atom or analkyl group of 1 to 4 carbons), —O— or an arylene group; the letters h,i and j are each independently 0 or an integer from 1 to 10; and theletter q is an integer which is 1 or higher.

The unsaturated alcohol serving as component (A) is not subject to anyparticular limitation, provided the molecule bears at least one(meth)acryloyl group and a hydroxyl group. Illustrative examples include2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethylmethacrylate, 2-hydroxylpropyl methacrylate, diethylene glycolmonoacrylate, diethylene glycol monomethacrylate, triethylene glycolmonoacrylate and triethylene glycol monomethacrylate.

The polyol compound serving as component (B) may be, for example, apolyether polyol such as polyethylene glycol or a polyester polyol suchas polycaprolactone. A polyol compound of general formula (15) above isespecially preferred.

In above formula (15), R⁵ and R⁶ are each independently a divalenthydrocarbon group of 1 to 10 carbons, and preferably 1 to 6 carbons,which may contain an amino, nitro, carbonyl or ether group. Preferredexamples include alkylene groups such as methylene, ethylene,trimethylene, propylene, ethylene oxide and propylene oxide groups.

The letter q is a number which is ≧1, preferably ≧5, and most preferablyfrom 10 to 200.

The polyol compound serving as component (B) has a number-averagemolecular weight of preferably 400 to 10,000, and more preferably 1,000to 5,000.

Illustrative examples of the polyisocyanate compound serving ascomponent (C) include aromatic diisocyanates such as tolylenediisocyanate, 4,4′-diphenylmethane diisocyanate, p-phenylenediisocyanate, 1,5-naphthylene diisocyanate,3,3′-dichloro-4,4′-diphenylmethane diisocyanate and xylylenediisocyanate; and aliphatic or alicyclic diisocyanates such ashexamethylene diisocyanate, isophorone diisocyanate,4,4′-dichlorohexylmethane diisocyanate and hydrogenated xylylenediisocyanate.

The above polyurethane compound is preferably one prepared from abovecomponents (A) to (C) and also a chain extender (D). Any chain extendercommonly used in the preparation of thermoplastic polyurethane resinsmay be employed. Illustrative examples include glycols such as ethyleneglycol and diethylene glycol; aliphatic diols such as 1,3-propanedioland 1,4-butanediol; aromatic or alicyclic diols such as1,4-bis(β-hydroxyethoxy)benzene, 1,4-cyclohexanediol and xylyleneglycol; diamines such as hydrazine, ethylenediamine,hexamethylenediamine, xylylenediamine and piperazine; and amino alcoholssuch as adipoyl hydrazide and isophthaloyl hydrazide. Any one orcombinations of two or more of these may be used.

Use may also be made of a urethane prepolymer prepared by thepreliminary reaction of the polyol compound serving as component (B)with the polyisocyanate compound serving as component (C).

It is advantageous to use components (A) to (D) in the followingproportions:

-   (A) 100 parts by weight;-   (B) 100 to 20,000 parts by weight, and preferably 1,000 to 10,000    parts by weight;-   (C) 80 to 5,000 parts by weight, and preferably 300 to 2,000 parts    by weight; and optionally,-   (D) 5 to 1,000 parts by weight, and preferably 10 to 500 parts by    weight.

The resulting unsaturated polyurethane compound has a number-averagemolecular weight of preferably 1,000 to 50,000, and most preferably3,000 to 30,000. Too small a number-average molecular weight results ina small molecular weight between crosslink sites in the cured gel, whichmay give it insufficient flexibility as a binder polymer. On the otherhand, a number-average molecular weight that is too large may cause theviscosity of the electrode composition prior to curing to become solarge as to make it difficult to fabricate an electrode having a uniformcoat thickness.

The above-mentioned polymeric material having an interpenetratingnetwork structure or semi-interpenetrating network structure (II) may becomposed of two or more compounds, such as polymers or reactivemonomers, which are capable of forming a mutually interpenetrating orsemi-interpenetrating network structure.

Examples of such polymeric materials and the two or more compounds ofwhich they are composed include:

-   (A) a polymer matrix formed by combining (a) a hydroxyalkyl    polysaccharide derivative with (d) a crosslinkable functional    group-bearing compound;-   (B) a polymer matrix formed by combining (b) a polyvinyl alcohol    derivative with (d) a crosslinkable functional group-bearing    compound; and-   (C) a polymer matrix formed by combining (c) a polyglycidol    derivative with (d) a crosslinkable functional group-bearing    compound.

Use of the above-described unsaturated polyurethane compound (I) as partor all of the crosslinkable functional group-bearing compound (d) isadvantageous for improving physical strength and other reasons.

Any of the following may be used as the hydroxyalkyl polysaccharidederivative serving as component (a):

-   (1) hydroxyethyl polysaccharides prepared by reacting ethylene oxide    with a naturally occurring polysaccharide such as cellulose, starch    or pullulan,-   (2) hydroxypropyl polysaccharides prepared by reacting propylene    oxide with the above naturally occurring polysaccharide,-   (3) dihydroxypropyl polysaccharides prepared by reacting glycidol or    3-chloro-1,2-propanediol with the above naturally occurring    polysaccharide.    Some or all of the hydroxyl groups on these hydroxyalkyl    polysaccharides may be capped with an ester-bonded or ether-bonded    substituent.

The above hydroxyalkyl polysaccharides have a molar degree ofsubstitution of 2 to 30, and preferably 2 to 20. At a molar substitutionbelow 2, the ability of the hydroxyalkyl polysaccharide to solvateelectrolyte salts becomes so low as to make the hydroxyalkylpolysaccharide unsuitable for use.

The hydroxyalkyl polysaccharide derivative in which some or all of thehydroxyl groups have been capped with ester-bonded or ether-bondedsubstituents may be one in which at least 10% of the terminal OH groupson the molecular chains have been capped with one or more type ofmonovalent group selected from among halogen atoms, substituted orunsubstituted monovalent hydrocarbon groups, R⁸CO— groups (wherein R⁸ isa substituted or unsubstituted monovalent hydrocarbon group), R⁸ ₃Si—groups (wherein R⁸ is the same as above), amino groups, alkylaminogroups, H(OR⁹)_(m)— groups (wherein R⁹ is an alkylene group of 2 to 5carbons and the letter m is an integer from 1 to 100), andphosphorus-containing groups.

The substituted or unsubstituted monovalent hydrocarbon groups are likeor unlike monovalent hydrocarbon groups having 1 to 10 carbons, andpreferably 1 to 8 carbons. Illustrative examples include alkyls such asmethyl, ethyl, propyl, isopropyl, t-butyl and pentyl; aryls such asphenyl and tolyl; aralkyls such as benzyl; alkenyls such as vinyl; andany of the foregoing groups in which some or all of the hydrogen atomshave been substituted with halogen, cyano, hydroxyl, amino or othersubstituents. Any one or combinations of two or more such groups may beused.

The polyvinyl alcohol derivative serving as component (b) is a polymericcompound having oxyalkylene chain-bearing polyvinyl alcohol units inwhich some or all of the hydroxyl groups are substituted. Here,“hydroxyl groups” refers collectively to residual hydroxyl groupsoriginating from the polyvinyl alcohol units and hydroxyl groups on theoxyalkylene-containing groups that have been introduced onto themolecule.

The polymeric compound having polyvinyl alcohol units has an averagedegree of polymerization of at least 20, preferably at least 30, andmost preferably at least 50. Some or all of the hydroxyl groups on thepolyvinyl alcohol units are substituted with oxyalkylene-containinggroups. For ease of handling and other reasons, the upper limit in thenumber-average degree of polymerization is preferably not higher than2,000, more preferably not higher than 500, and most preferably nothigher than 200.

It is most advantageous for the polyvinyl alcohol unit-containingpolymeric compound to be a homopolymer which satisfies the above rangein the number-average degree of polymerization and in which the fractionof polyvinyl alcohol units within the molecule is at least 98 mol %.However, use can also be made of polyvinyl alcohol unit-containingpolymeric compounds which satisfy the above range in the number-averagedegree of polymerization and have a polyvinyl alcohol fraction ofpreferably at least 60 mol %, and more preferably at least 70 mol %.Illustrative examples include polyvinylformal in which some of thehydroxyl groups on the polyvinyl alcohol have been converted to formal,modified polyvinyl alcohols in which some of the hydroxyl groups on thepolyvinyl alcohol have been alkylated, poly(ethylene vinyl alcohol),partially saponified polyvinyl acetate, and other modified polyvinylalcohols.

Some or all of the hydroxyl groups on the polyvinyl alcohol units of thepolymeric compound are substituted with oxyalkylene-containing groups(moreover, some of the hydrogen atoms on these oxyalkylene groups may besubstituted with hydroxyl groups) to an average molar substitution of atleast 0.3. The proportion of hydroxyl groups substituted withoxyalkylene-containing groups is preferably at least 30 mol %, and morepreferably at least 50 mol %. The average molar substitution (MS) can bedetermined by accurately measuring the weight of the polyvinyl alcoholcharged and the weight of the reaction product.

The polyglycidol derivative serving as component (c) is a compoundcontaining units of formula (16) below (referred to hereinafter as “Aunits”)

and units of formula (17) (referred to hereinafter as “B units”)

The ends of the molecular chains on the compound are capped withspecific substituents.

The polyglycidol can be prepared by polymerizing glycidol or3-chloro-1,2-propanediol, although it is generally advisable to carryout polymerization using glycidol as the starting material, and using abasic catalyst or a Lewis acid catalyst.

The total number of A and B units on the polyglycidol molecule ispreferably at least two, more preferably at least six, and mostpreferably at least ten. There is no particular upper limit, althoughthe total number of such groups generally is not more than about 10,000.The total number of these units may be set as appropriate for therequired flowability, viscosity and other properties of thepolyglycidol. The ratio of A units to B units (A:B) in the molecule iswithin a range of preferably 1/9 to 9/1, and especially 3/7 to 7/3. TheA and B units do not appear in a regular order, and may be arranged inany combination.

The polyglycidol has a polyethylene glycol equivalent weight-averagemolecular weight (Mw), as determined by gel permeation chromatography(GPC), within a range of preferably 200 to 730,000, more preferably 200to 100,000, and most preferably 600 to 20,000. The polydispersity(Mw/Mn) is preferably 1.1 to 20, and most preferably 1.1 to 10.

The polyglycidol in which the molecular chains are end-capped withsubstituents is a polyglycidol derivative in which at least 10% of theterminal hydroxyl groups on the molecular chains are capped with one ormore type of monovalent group selected from among halogen atoms,substituted or unsubstituted monovalent hydrocarbon groups, R¹⁰CO—groups of 1 to 10 carbons (wherein R¹⁰ is a substituted or unsubstitutedmonovalent hydrocarbon group), R¹⁰ ₃Si— groups (wherein R¹⁰ is asdefined above), amino groups, alkylamino groups, H(OR¹¹)_(u)— groups(wherein R¹¹ is an alkylene group of 2 to 5 carbons, and the letter u isan integer from 1 to 100), and phosphorus atom-containing groups.

The foregoing substituted or unsubstituted monovalent hydrocarbon groupsof 1 to 10 carbons are exemplified by the same groups as those mentionedabove for R⁸ and R⁹. Such groups having 1 to 8 carbons are especiallypreferred. Substitution may be carried out by using known techniques forintroducing various substituents at hydroxyl end groups.

Any of the following may be used as the crosslinkable functionalgroup-bearing compound serving as component (d):

-   (1) an epoxy group-bearing compound in combination with a compound    having two or more active hydrogens capable of reacting with the    epoxy group;-   (2) an isocyanate group-bearing compound in combination with a    compound having two or more active hydrogens capable of reacting    with the isocyanate group;-   (3) a compound having two or more reactive double bonds.

Preferred examples of the epoxy group-bearing compound (1) includecompounds having two or more epoxy groups on the molecule, such assorbitol polyglycidyl ether, sorbitan polyglycidyl ether, polyglycerolpolyglycidyl ether, pentaerythritol polyglycidyl ether, diglycerolpolyglycidyl ether and triglycidyl tris(2-hydroxyethyl) isocyanurate.

A three-dimensional network structure can be formed by reacting theabove epoxy group-bearing compound with a compound having at least twoactive hydrogens, such as an amine, alcohol, carboxylic acid or phenol.Illustrative examples include polymeric polyols such as polyethyleneglycol, polypropylene glycol and ethylene glycol-propylene glycolcopolymers, and also ethylene glycol, 1,2-propylene glycol,1,3-propylene glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol,2,2-dimethyl-1,3-propanediol, diethylene glycol, dipropylene glycol,1,4-cyclohexanedimethanol, 1,4-bis(β-hydroxyethoxy)benzene, andp-xylylenediol; polyamines such as phenyl diethanolamine, methyldiethanolamine and polyethyleneimine; and polycarboxylic acids.

Illustrative examples of the isocyanate group-bearing compound (2)include compounds having two or more isocyanate groups, such as tolylenediisocyanate, xylylene diisocyanate, naphthylene diisocyanate,diphenylmethane diisocyanate, biphenylene diisocyanate, diphenyl etherdiisocyanate, tolidine diisocyanate, hexamethylene diisocyanate andisophorone diisocyanate.

An isocyanato-terminal polyol compound prepared by reacting the aboveisocyanate compound with a polyol compound can also be used.

In this case, the stoichiometric ratio between the isocyanate groups[NCO] on the isocyanate compound and the hydroxyl groups [OH] on thepolyol compound is such as to satisfy the condition [NCO]>[OH]. Theratio [NCO]/[OH] is preferably in a range of 1.03/1 to 10/1, andespecially 1.10/1 to 5/1.

Alternatively, instead of the polyol, an amine having two or more activehydrogens may be reacted with the isocyanate. The amine used may be onehaving a primary or a secondary amino group, although a primary aminogroup-bearing compound is preferred. Suitable examples include diaminessuch as ethylenediamine, 1,6-diaminohexane, 1,4-diaminobutane andpiperazine; polyamines such as polyethyleneamine; and amino alcoholssuch as N-methyldiethanolamine and aminoethanol. Of these, diamines inwhich the functional groups have the same level of reactivity areespecially preferred. Here too, the stoichiometric ratio between [NCO]groups on the isocyanate compound and [NH₂] or [NH] groups on the aminecompound is such as to satisfy the condition [NCO]>[NH₂]+[NH].

The above isocyanate group-bearing compounds cannot by themselves formthree-dimensional network structures. However, three-dimensional networkstructures can be formed by reacting the isocyanate group-bearingcompound with a compound having at least two active hydrogens, such asan amine, alcohol, carboxylic acid or phenol.

Suitable compounds having two or more active hydrogens are exemplifiedby the same compounds as those mentioned above.

The aforementioned reactive double bond-bearing compound (3) is notsubject to any particular limitation, although preferred examplesinclude the above-described unsaturated polyurethane compounds (I) andpolyoxyalkylene component-bearing diesters of general formula (18)below. The use of these in combination with a polyoxyalkylenecomponent-bearing monoester of general formula (19) below and a triesteris recommended.

In formula (18), R¹², R¹³ and R¹⁴ are each independently a hydrogen atomor an alkyl group having 1 to 6 carbons, and preferably 1 to 4 carbons,such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl andt-butyl; and the letters d and e satisfy the condition d≧1 and e≧0 orthe condition d≧0 and e≧1. The sum d+e is preferably no higher than 100,and especially from 1 to 30. R¹², R¹³ and R¹⁴ are most preferablymethyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl or t-butyl.

In formula (19), R¹⁵, R¹⁶ and R¹⁷ are each independently a hydrogen atomor an alkyl group having 1 to 6 carbons, and preferably 1 to 4 carbons,such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl andt-butyl; and the letters f and g satisfy the condition f≧1 and g≧0 orthe condition f≧0 and g≧1. The sum f+g is preferably no higher than 100,and especially from 1 to 30. R¹⁵, R¹⁶ and R¹⁷ are most preferablymethyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl or t-butyl.

If necessary, a compound containing an acrylic or methacrylic group maybe added. Examples of such compounds include acrylates and methacrylatessuch as glycidyl methacrylate, glycidyl acrylate and tetrahydrofurfurylmethacrylate, as well as methacryloyl isocyanate,2-hydroxymethylmethacrylic acid and N,N-dimethylaminoethylmethacrylicacid. Other reactive double bond-containing compounds may be added aswell, such as acrylamides (e.g., N-methylolacrylamide,methylenebisacrylamide, diacetoneacrylamide), and vinyl compounds suchas vinyloxazolines and vinylene carbonate.

Here too, in order to form a three-dimensional network structure, acompound having at least two reactive double bonds like those mentionedabove must be added.

Typically, the above-described unsaturated polyurethane compound (I) orpolyoxyalkylene component-bearing diester compound and thepolyoxyalkylene component-bearing monoester compound are heated orexposed to a suitable form of radiation, such as electron beams,microwaves or radio-frequency radiation, within the electrodecomposition, or a mixture of the compounds is heated, so as to form thethree-dimensional network structure.

The addition of a polyoxyalkylene component-bearing monoester compound,which is a monofunctional monomer, to the unsaturated polyurethanecompound or the polyoxyalkylene component-bearing diester compound isdesirable because such addition introduces polyoxyalkylene branchedchains onto the three-dimensional network.

No particular limitation is imposed on the relative proportions of theunsaturated polyurethane compound or polyoxyalkylene component-bearingdiester compound and the polyoxyalkylene component-bearing monoestercompound.

The binder polymer containing component (a), (b) or (c) in combinationwith component (d), when heated or exposed to a suitable form ofradiation, such as electron beams, microwaves or radio-frequencyradiation, forms a semi-interpenetrating polymer network structure inwhich molecular chains of a polymer of component (a), (b) or (c) areinterlocked with the three-dimensional network structure of a polymerformed by the reaction (polymerization) of the crosslinkable functionalgroup-bearing compound serving as component (d).

Thermoplastic resins containing units of general formula (14) below maybe used as the above-mentioned type (III) binder polymer.

In the formula, the letter r is 3, 4 or 5, and the letter s is aninteger ≧5.

Such a thermoplastic resin is preferably a thermoplastic polyurethaneresin prepared by reacting (E) a polyol compound with (F) apolyisocyanate compound and (G) a chain extender.

Suitable thermoplastic polyurethane resins include not only polyurethaneresins having urethane linkages, but also polyurethane-urea resinshaving both urethane linkages and urea linkages.

Preferred examples of the polyol compound serving as component (E) aboveinclude polyester polyol, polyester polyether polyol, polyesterpolycarbonate polyol, polycaprolactone polyol, and mixtures thereof.

The polyol compound serving as component (E) has a number-averagemolecular weight of preferably 1,000 to 5,000, and most preferably 1,500to 3,000. A polyol compound having too small a number-average molecularweight may lower the physical properties of the resulting thermoplasticpolyurethane resin film, such as the heat resistance and tensileelongation. On the other hand, too large a number-average molecularweight increases the viscosity during synthesis, which may lower theproduction stability of the thermoplastic polyurethane resin beingprepared. The number-average molecular weights used here in connectionwith polyol compounds are calculated based on the hydroxyl valuesmeasured in accordance with JIS K1577.

Illustrative examples of the polyisocyanate compound serving as abovecomponent (F) include aromatic diisocyanates such as tolylenediisocyanate, 4,4′-diphenylmethane diisocyanate, p-phenylenediisocyanate, 1,5-naphthylene diisocyanate and xylylene diisocyanate;and aliphatic or alicyclic diisocyanates such as hexamethylenediisocyanate, isophorone diisocyanate, 4,4′-dicyclohexylmethanediisocyanate and hydrogenated xylylene diisocyanate.

The chain extender serving as above component (G) is preferably alow-molecular-weight compound having a molecular weight of not more than300 and bearing two active hydrogen atoms capable of reacting withisocyanate groups.

Various types of known compounds may be used as suchlow-molecular-weight compounds. Illustrative examples include aliphaticdiols such as ethylene glycol, propylene glycol and 1,3-propanediol;aromatic or alicyclic diols such as 1,4-bis(β-hydroxyethoxy)benzene,1,4-cyclohexanediol and bis(β-hydroxyethyl) terephthalate; diamines suchas hydrazine, ethylenediamine, hexamethylenediamine and xylylenediamine;and amino alcohols such as adipoyl hydrazide. Any one or combinations oftwo or more of these may be used.

In preparing the thermoplastic polyurethane resin, it is advantageous toreact the above components in the following proportions:

-   (E) 100 parts by weight of the polyol compound;-   (F) 5 to 200 parts by weight, and preferably 20 to 100 parts by    weight, of the polyisocyanate compound;-   (G) 1 to 200 parts by weight, and preferably 5 to 100 parts by    weight, of the chain extender.

The thermoplastic resin has a swelling ratio, as determined from theformula indicated below, within a range of 150 to 800%, preferably 250to 500%, and most preferably 250 to 400%.

${{Swelling}{\;\mspace{11mu}}{ratio}\mspace{14mu}(\%)} = {\frac{\begin{matrix}{{weight}\mspace{14mu}{in}\mspace{14mu}{grams}{\;\mspace{11mu}}{of}\mspace{14mu}{swollen}\mspace{14mu}{thermoplastic}} \\{\mspace{14mu}{{resin}\mspace{14mu}{after}\mspace{14mu} 24\text{-}{hour}{\mspace{11mu}\;}{immersion}}} \\{\mspace{14mu}{{in}\mspace{14mu}{electrolyte}\mspace{14mu}{solution}{\mspace{11mu}\;}{at}\mspace{14mu} 20{{\,^{\circ}\mspace{14mu} C}.\mspace{11mu}(g)}}}\end{matrix}}{\begin{matrix}{{weight}\mspace{14mu}{in}\mspace{14mu}{grams}{\mspace{11mu}\;}{of}\mspace{14mu}{thermoplasic}\mspace{11mu}{resin}\mspace{14mu}{before}} \\{{\mspace{11mu}\;}{{immersion}\mspace{14mu}{in}\mspace{14mu}{electrolyte}}\mspace{11mu}} \\{{solution}{\mspace{11mu}\;}{at}{\mspace{11mu}\;}20{{\,^{\circ \mspace{14mu}}C}.\mspace{14mu}(g)}}\end{matrix}} \times 100}$

Preferred examples of fluoropolymer materials that may be used as theabove-mentioned type (IV) binder polymer include polyvinylidene fluoride(PVDF), vinylidene fluoride-hexafluoropropylene copolymers (P(VDF-HFP))and vinylidene fluoride-chlorotrifluoroethylene copolymers(P(VDF-CTFE)). Of these, fluoropolymers having a vinylidene fluoridecontent of preferably at least 50 wt %, and most preferably at least 70wt %, are especially desirable. The upper limit in the vinylidenefluoride content of the fluoropolymer is preferably about 97 wt %.

No particular limitation is imposed on the weight-average molecularweight of the fluoropolymer, although the weight-average molecularweight is preferably from 500,000 to 2,000,000, and most preferably from500,000 to 1,500,000. Too low a weight-average molecular weight mayresult in an excessive decline in physical strength.

The polarizable electrode composition can be produced by charging amixer with a binder solution prepared from the above-describedcarbonaceous material (which includes, if necessary, a conductivematerial), a binder polymer and, optionally, a solvent, then wet mixing.

The amount of binder polymer added is preferably 0.5 to 20 parts byweight, and most preferably 1 to 10 parts by weight, per 100 parts byweight of the carbonaceous material.

The polarizable electrode composition prepared as described above iscoated onto a current conductor, thereby forming a polarizableelectrode. Any positive and negative electrode current collectorscommonly used in electrical double-layer capacitors may be selected andused, although the positive electrode current collector is preferablyaluminum foil or aluminum oxide and the negative electrode currentcollector is preferably copper foil, nickel foil, or a metal foil whosesurface is formed of a film of plated copper or nickel.

The foils making up the respective current collectors may be in any ofvarious shapes, including thin foils, flat sheets, and perforated,stampable sheets. The foil has a thickness of generally about 1 to 200μm. For optimal characteristics, such as density of the carbonaceousmaterial as a portion of the overall electrode and electrode strength, athickness of 8 to 100 μm, and especially 8 to 30 μm, is preferred.

The polarizable electrode can be produced by melting and blending thepolarizable electrode composition, then extruding the blend as a film.

The separator referred to above may be of a type that is commonly usedin electrical double-layer capacitors. Illustrative examples include (1)separators produced by impregnating a separator base with a liquidelectrolyte, (2) separators produced by shaping the polymer binder usedin the polarizable electrode as a film, and (3) separators composed of agel electrolyte film produced by shaping a thermoplastic resin having aswelling ratio, as determined by the formula indicated above, within arange of 150 to 800%, then impregnating the resin with a liquidelectrolyte so as to induce it to swell. The liquid electrolyte used forthis purpose may be any of the various types of above-mentioned liquidelectrolytes for electrical storage devices.

The separator base used in type (1) separators may be one that iscommonly used in electrical double-layer capacitors. Illustrativeexamples include polyolefin nonwoven fabric, polytetrafluoroethyleneporous film, kraft paper, sheet laid from a blend of rayon fibers andsisal fibers, manila hemp sheet, glass fiber sheet, cellulose-basedelectrolytic paper, paper made from rayon fibers, paper made from ablend of cellulose and glass fibers, and combinations thereof in theform of multilayer sheets.

Other types of separators that may be used include (2) separatorsproduced by shaping the polymer binder used in the polarizableelectrodes as a film, and (3) separators composed of a gel electrolytefilm obtained by shaping a thermoplastic resin having a swelling ratio,as determined by the formula indicated above, within a range of 150 to800%, then impregnating the resin with a liquid electrolyte so as toinduce it to swell.

Because such separators have the same composition as the polymer binder(thermoplastic resin) used in the electrodes, the electrode/separatorinterface can be integrally united and controlled, making it possible tofurther lower the internal resistance of the capacitor.

The electrical double-layer capacitor of the invention can be assembledby stacking, fan-folding or winding an electrical double-layer capacitorassembly composed of a pair of polarizable electrodes produced asdescribed above and a separator therebetween. The cell assembly isformed into a coin-like shape, then placed within a capacitor housingsuch as a can or a laminate pack. The assembly is then filled with theliquid electrolyte, following which the housing is mechanically sealedif it is a can or heat-sealed if it is a laminate pack.

Because the electrical double-layer capacitors of the invention use thequaternary ammonium salt or quaternary phosphonium salt of generalformula (1) above as the electrolyte, the ionic conductivity is higherthan in prior-art electrical double-layer capacitors, in addition towhich the capacitors have a high electrostatic capacitance, excellentlow-temperature characteristics and a broad potential window. Moreover,the use of low-impedance polarizable electrodes like those describedabove makes it possible to endow the capacitor with a high power densityand energy density.

Because they are endowed with such characteristics, the electricaldouble-layer capacitors of the invention are highly suitable for use asa memory backup power supply for cellular phones, notebook computers andwireless terminals, as a power supply for cellular phones and portableacoustic devices, as an uninterruptible power supply for personalcomputers and other equipment, and as various types of low-currentelectrical storage devices such as load leveling power supplies used incombination with solar power generation and wind power generation.Moreover, electrical double-layer capacitors capable of being chargedand discharged at a high current are highly suitable for use ashigh-current electrical storage devices in such applications as electriccars and electrical power tools.

Secondary Batteries

The secondary battery of the invention has a positive electrode and anegative electrode, a separator between the positive and negativeelectrodes, and an electrolyte solution. The electrolyte solution is anyof the above-mentioned liquid electrolytes for electrical storagedevices to which has been added an ion-conductive salt (liquidelectrolytes (2) and (3) described under Liquid Electrolytes forElectrical Storage Devices).

The positive electrode active material making up the positive electrodeis suitably selected in accordance with the intended use of theelectrode, the type of battery and other considerations. For example, inthe case of positive electrodes in lithium secondary cells and lithiumion secondary cells, use can be made of chalcogen compounds capable ofoccluding and releasing lithium ions, and lithium ion-containingchalcogen compounds.

Examples of such chalcogen compounds capable of occluding and releasinglithium ions include FeS₂, TiS₂, MoS₂, V₂O₆, V₆O₁₃ and MnO₂.

Specific examples of lithium ion-containing chalcogen compounds includeLiCoO₂, LiMnO₂, LiMn₂O₄, LiMo₂O₄, LiV₃O₈, LiNiO₂ andLi_(x)Ni_(y)M_(1-y)O₂ (wherein M is one or more metal element selectedfrom among cobalt, manganese, titanium, chromium, vanadium, aluminum,tin, lead and zinc; 0.05≦x≦1.10; and 0.5≦y≦1.0).

The negative electrode active material making up the negative electrodeis suitably selected in accordance with the intended use of theelectrode, the type of battery and other considerations. For example, inthe case of negative electrodes in lithium secondary cells and lithiumion secondary cells, use can be made of alkali metals, alkali metalalloys, oxides, sulfides or nitrides of at least one element selectedfrom among group 8, 9, 10, 11, 12, 13, 14 and 15 elements of theperiodic table capable of reversibly occluding and releasing lithiumions, and carbonaceous materials capable of occluding and releasinglithium ions.

Examples of suitable alkali metals include lithium, sodium andpotassium. Examples of suitable alkali metal alloys include metalliclithium, Li—Al, Li—Mg, Li—Al—Ni, sodium, Na—Hg and Na—Zn.

Illustrative examples of the oxides of at least one element selectedfrom periodic table group 8 to 15 elements capable of occluding andreleasing lithium ions include tin silicon oxide (SnSiO₃), lithiumbismuth oxide (Li₃BiO₄) and lithium zinc oxide (Li₂ZnO₂).

Illustrative examples of the sulfides include lithium iron sulfidesLi_(x)FeS₂ (wherein 0≦x≦3) and lithium copper sulfides Li_(x)CuS(wherein 0≦x≦3).

Illustrative examples of the nitrides include lithium-containingtransition metal nitrides, and specifically Li_(x)M_(y)N (wherein M iscobalt, nickel or copper; 0≦x≦3; and 0≦y≦0.5) and lithium iron nitride(Li₃FeN₄).

Examples of carbonaceous materials which are capable of reversiblyoccluding and releasing lithium ions include graphite, carbon black,coke, glassy carbon, carbon fibers, and sintered bodies obtained fromany of these.

The binder polymer and separator which make up the positive and negativeelectrodes are the same as those described above for electricaldouble-layer capacitors. Ion-conductive salts that may be used are theconductive salts described above under Liquid Electrolytes forElectrical Storage Devices.

The secondary battery described above can be assembled by stacking,fan-folding or winding a cell assembly composed of a positive electrodeand a negative electrode with a separator therebetween. The cellassembly is formed into a coin-like shape, then placed within a batteryhousing such as a can or a laminate pack. The assembly is then filledwith the electrolyte solution, following which the housing ismechanically sealed if it is a can or heat-sealed if it is a laminatepack.

If necessary, a reaction-curable substance such as a (meth)acrylate, anepoxy group-bearing compound or a heat-curable urethane can be added tothe electrolyte solution and a reaction carried out to effect curing.

Electrolyte solutions (2) and (3) described above under LiquidElectrolytes can also be used in hybrid-type electrical storage devicesin which the positive or negative electrode is a polarizable electrodesuch as is commonly used in electrical double-layer capacitors and theother, opposing, electrode is an electrode in which the active materialis a substance capable of the insertion and extraction of lithium ions,such as is commonly used in lithium ion secondary batteries.

EXAMPLE

Synthesis examples, examples of the invention and comparative examplesare given below to more fully illustrate the invention, and are notintended to limit the scope thereof.

Synthesis Example 1

Synthesis of Compound (3)

A mixed solution prepared by mixing together 100 ml of diethylamine(Kanto Chemical Co., Inc.) and 85 ml of 2-methoxyethyl chloride (KantoChemical) was placed in an autoclave and reacted at 100° C. for 24hours. The internal pressure during the reaction was 1.3 kgf/cm². After24 hours, 200 ml of an aqueous solution containing 56 g of potassiumhydroxide (Katayama Chemical Inc.) was added to the resulting mixture ofdeposited crystals and reaction solution. The two organic phases thatformed as a result were separated off with a separatory funnel andsubjected twice to extraction with 100 ml of methylene chloride (WakoPure Chemical Industries, Ltd.). The separated organic phases were thencombined and washed with a saturated saline solution, following whichpotassium carbonate (Wako Pure Chemical Industries) was added to removewater, and vacuum filtration was carried out. The solvent in theresulting organic phase was distilled off in a rotary evaporator,following which the residue was subjected to normal-pressuredistillation, yielding 18.9 g of a fraction that boiled at about 135° C.This compound was confirmed from the ¹H-NMR spectrum to be2-methoxyethyldiethylamine.

Next, 8.24 g of the 2-methoxyethyldiethylamine was dissolved in 10 ml oftetrahydrofuran (Wako Pure Chemical Industries), following which 4.0 mlof methyl iodide (Wako Pure Chemical Industries) was added under icecooling. After 30 minutes, the mixture was removed from the ice bath andstirred overnight at room temperature. The solvent in the resultingreaction mixture was then driven off by vacuum distillation, and theresulting solids were recrystallized from an ethanol (Wako Pure ChemicalIndustries)—tetrahydrofuran system, yielding 16 g of2-methoxyethyldiethylmethylammonium iodide.

Next, 15.0 g of the 2-methoxyethyldiethylmethylammonium iodide wasdissolved in 100 ml of distilled water, following which 6.37 g of silveroxide (Kanto Chemical) was added and stirring was carried out for 3hours. The reaction mixture was then vacuum filtered to remove theprecipitate, following which 42% tetrafluoroboric acid (Kanto Chemical)was gradually added under stirring until the reaction solution reached apH of about 5 to 6. The reaction solution was subsequently freeze-dried,in addition to which water was thoroughly driven off using a vacuumpump, ultimately yielding 12.39 g of a compound (3) that was liquid atroom temperature (25° C.).

FIG. 1 shows the NMR spectrum (solvent: deuterated chloroform) forcompound (3).

Synthesis Example 2

Synthesis of Compound (4)

Aside from using ethyl iodide instead of methyl iodide, compound (4) ofthe above formula was synthesized in the same way as in SynthesisExample 1. The white crystals obtained after freeze-drying wererecrystallized from ethanol to give a pure product.

FIG. 2 shows the NMR spectrum (solvent: deuterated chloroform) forcompound (4).

Synthesis Example 3

Synthesis of Compound (5)

Aside from using pyrrolidine instead of diethylamine and setting thereaction temperature in the autoclave at 90° C., compound (5) of theabove formula was synthesized in the same way as in Synthesis Example 1.The target substance was a liquid at room temperature (25° C.).

FIG. 3 shows the NMR spectrum (solvent: deuterated chloroform) forcompound (5).

Synthesis Example 4

Synthesis of Compound (6)

Aside from using piperazine instead of diethylamine and setting thereaction temperature in the autoclave at 100° C., compound (6) of theabove formula was synthesized in the same way as in Synthesis Example 1.The target substance was a liquid at room temperature (25° C.).

FIG. 4 shows the NMR spectrum (solvent: deuterated chloroform) forcompound (6).

Synthesis Example 5

Synthesis of Compound (7)

First, 200 ml of a toluene solution of triethylphosphine(triethylphosphine content, approx. 20%; product of Kanto Chemical) wasmixed with 50 ml of 2-methoxyethyl chloride (Kanto Chemical) to effect areaction, which was carried out under refluxing for 24 hours. Thesolvent was then distilled off at normal pressure, following which theremaining solvent and unreacted reagents were completed removed bydistillation using a vacuum pump. The residue was recrystallized from anethanol-THF system, yielding 45 g of 2-methoxyethyltriethylphosphoniumchloride.

Next, 20.0 g of the 2-methoxyethyltriethylphosphonium chloride thusobtained was dissolved in 100 ml of distilled water, following which10.89 g of silver oxide (Kanto Chemical) was added and the mixture wasstirred for 2 hours. The precipitate was then removed by vacuumfiltration, following which 42% tetrafluoroboric acid (Kanto Chemical)was gradually added under stirring until the reaction solution reached apH of about 5 to 6. The reaction solution was subsequently freeze-dried,in addition to which water was thoroughly driven off using a vacuumpump, yielding 23.87 g of a compound (7) that was liquid at roomtemperature (25° C.).

Synthesis Example 6

Synthesis of Compound (8)

First, 10.0 g of 2-methoxyethyldiethylmethylammonium iodide obtained bythe same method as in Synthesis Example 1 was dissolved in 50 mL ofacetonitrile (Kanto Chemical). Next, 9.5 g of lithiumbis(trifluoromethanesulfonyl)imide (produced by Kishida Chemical Co.,Ltd.) was added and completely dissolved therein, following which thesolution was stirred for 15 minutes.

The acetonitrile was removed by vacuum distillation, then water wasadded to the residue, causing the organic phase to separate into two.The organic phase was then separated off and washed five times withwater to remove impurities.

The washed organic phase was subsequently placed under reduced pressurewith a vacuum pump and the water was thoroughly driven off, yielding 6.8g of compound (8) that was liquid at room temperature.

FIG. 5 shows the NMR spectrum (solvent: deuterated chloroform) forcompound (8).

Synthesis Example 7

Synthesis of Compound (9)

First, 10.0 g of 2-methoxyethyldiethylmethylammonium iodide obtained bythe same method as in Synthesis Example 1 was dissolved in 50 mL ofacetonitrile (Kanto Chemical). Next, 9.26 g of silverhexafluorophosphate (supplied by Aldrich Chemical Co., Ltd.) was addedand the mixture was stirred for one hour.

The reaction mixture was Celite filtered to remove the solids thereinand the solvent was driven off, following which the residue wasthoroughly dried under a vacuum, yielding 10.1 g of compound (9) thatwas liquid at room temperature.

FIG. 6 shows the NMR spectrum (solvent: deuterated chloroform) forcompound (9).

Synthesis Example 8

Synthesis of Compound (10)

Aside from substituting silver trifluoromethane sulfonate (AldrichChemical) for silver hexafluorophosphate and adding the silvertrifluoromethane sulfonate in an amount that is equimolar with the2-methoxyethyldiethylmethylammoniuom iodide, a compound (10) that isliquid at room temperature (25° C.) was obtained by the same method asin Synthesis Example 7.

FIG. 7 shows the NMR spectrum (solvent: deuterated chloroform) forcompound (10).

Synthesis Example 9

Synthesis of Compound (11)

Aside from using chloroform (Wako Pure Chemical Industries Ltd.) insteadof acetonitrile as the solvent, using silver trifluoroacetate (AldrichChemical) instead of silver hexafluorophosphate, and adding the silvertrifluoroacetate in an amount that is equimolar with the2-methoxyethyldiethylmethylammoniuom iodide, a compound (11) that wasliquid at room temperature (25° C.) was obtained by the same method asin Synthesis Example 7.

FIG. 8 shows the NMR spectrum (solvent: deuterated chloroform) forcompound (11).

Examples 1 to 5

Electrical Double-Layer Capacitors

The electrolyte salts prepared in Synthesis Examples 1 to 5 were eachdissolved in propylene carbonate (PC) to a concentration of 2.0 M, andthe resulting electrolyte solutions were used to manufacture electricaldouble-layer capacitors in the manner described below.

First, the activated carbon MSP-20 (Kansai Netsukagaku K.K.), analkali-activated product made from phenolic resin, was mixed withconductive carbon, polyurethane resin and N-methylpyrrolidone (NMP) in aspecific ratio (activated carbon/conductive carbon/polyurethaneresin/NMP=41.9:3.7:2.2:52.2) to form a paste, thereby giving polarizableelectrode compositions for the positive and negative electrodes ofelectrical double-layer capacitors. The resulting paste-like polarizableelectrode compositions were applied onto an aluminum plate with a doctorblade to a dry film thickness of 100 μm, dried at 80° C. for a period of4 hours, then rolled, thereby giving polarizable electrodes. Each cellwas assembled by placing a cellulose-based separator between a pair ofthe polarizable electrodes. The respective above-described electrolytesolutions were then injected into the assembled cells, giving electricaldouble-layer capacitors.

Comparative Example 1

Aside from using tetraethylammonium tetrafluoroborate, which is commonlyemployed as an electrolyte salt for nonaqueous electrical double-layercapacitors, and using a saturated propylene carbonate solution of thiselectrolyte salt (concentration, about 1.5 M) as the electrolytesolution, electrical double-layer capacitors were manufactured in thesame way as in the foregoing examples according to the invention.

Comparative Example 2

Aside from using a solution of tetraethylammonium tetrafluoroboratedissolved in propylene carbonate to a concentration of 1 M as theelectrolyte solution, electrical double-layer capacitors weremanufactured in the same way as in the above examples of the invention.

Electrostatic Capacitance and Ionic Conductivity:

The electrical double-layer capacitors manufactured in the aboveexamples of the invention and the comparative examples were subjected toa current density charge-discharge test under the conditions shownbelow, from which the electrostatic capacitance was measured. The ionicconductivity at −20° C. was also measured.

Capacitance Measurement Conditions:

Each electrical double-layer capacitor was charged and discharged at acurrent density of 1.59 mA/cm² and a voltage setting of 2.0 to 2.5 V.The capacitor was charged at a constant current; once the voltagereached a predetermined value, charging was continued at that voltagelevel for at least two hours, following which discharge was carried outat a current density of 1.59 mA/cm². The capacitance was computed fromthe integrated value of the electrical energy at discharge. The resultsare given in Table 1 below.

TABLE 1 Electrolyte Ionic salt Capac- conductivity Electrolyteconcentration itance at −20° C. salt (M) (F/g) (mS/cm) Example 1Compound (3) 2.0 32.1 7.3 Example 2 Compound (4) 2.0 31.0 6.2 Example 3Compound (5) 2.0 33.5 6.8 Example 4 Compound (6) 2.0 31.8 5.8 Example 5Compound (7) 2.0 30.5 5.6 Comparative TEA Saturated 27.0 — Example 1(1.5) Comparative TEA 1.0 24.0 3.5 Example 2

As is apparent from Table 1, a higher capacitance was achieved inExamples 1 to 5 according to the invention, in which a quaternaryammonium salt or a quaternary phosphonium salt was used as theelectrolyte salt, than in the comparative examples.

Moreover, although the salt concentrations in Examples 1 to 5 werehigher than in the comparative examples, deposition of the electrolytesalt did not occur. As a result, the ionic conductivities were higherthan in Comparative Example 2, demonstrating the usefulness of thesecapacitors because a greater amount of electrical energy can be drawn atlow temperatures. In Comparative Example 1, the electrolyte salt settledout of the electrolyte solution, rendering measurement of the ionicconductivity impossible.

Example 6

Electrical Double-Layer Capacitor

The electrolyte salt prepared in Synthesis Example 1 was dissolved in amixed solvent of propylene carbonate and ethylene carbonate (PC/EC=9:1)to give an electrolyte solution having a concentration of 2.0 M. Next,two polarizable electrodes (8×16 cm) coated on both sides and twopolarizable electrodes (8×16 cm) coated on one side were fabricated.Nickel tab terminals were welded to the electrodes.

Electrical double-layer capacitors were test-built by assembling thepolarizable electrodes with the pair of double-sided electrodes stackedtogether in the middle and a single-sided electrode positioned over eachof the two outside surfaces thereof so as to form positive and negativeelectrodes, and packing these electrodes as a laminate. The resultingelectrical double-layer capacitor was subjected to a charge-dischargetest. The capacitance, as determined by the energy equivalence methodusing a discharge curve, was 180 F.

Example 7

Electrical Double-Layer Capacitor

(1) Production of Activated Carbon (from polycarbodiimide)

A polycarbodiimide solution was prepared by reacting 54 parts by weightof an 80/20 mixture of 2,4-tolylene diisocyanate and 2,6-tolylenediisocyanate in 500 parts by weight of tetrachloroethylene in thepresence of 0.12 part by weight of a carbodiimide catalyst(1-phenyl-3-methyl-phospholene oxide) at 120° C. for a period of 4hours. The solvent was then driven off by vacuum distillation, yieldinga highly viscous liquid polycarbodiimide resin.

The carbodiimide resin was treated at 300° C. for 5 hours and completelysolidified, following which it was carbonized by 1 hour of heattreatment at 800° C. The resulting carbide was subjected to steamactivation treatment at 900° C. involving the introduction of water atan hourly rate of 5 parts by weight per part by weight of carbide,thereby yielding 6.2 parts by weight of the desired activated carbon.

(2) Manufacture of Electrical Double-Layer Capacitor

Aside from preparing a 2.0 M electrolyte solution by dissolving theelectrolyte salt obtained in Synthesis Example 1 in propylene carbonate,and using the activated carbon produced as described above instead ofMSP-20 in the polarizable electrodes, an electrical double-layercapacitor was manufactured in the same way as in Example 6.

The resulting electrical double-layer capacitor was subjected to acharge-discharge test. The capacitance, as determined by the energyequivalence method using a discharge curve, was 178 F.

Example 8

Electrical Double-Layer Capacitor

Activated carbon (MSP-20, made by Kansai Netsukagaku K.K.), a conductivematerial (Denka Black HS100, made by Denki Kagaku Kogyo K.K.) and abinder (PVdF900, made by Kureha Chemical Industry Co., Ltd.) were usedas the filler substances in a respective weight ratio of 100:3:5 (basedon 100 parts by weight of the activated carbon). These fillers weremixed with N-methyl-2-pyrrolidone (NMP) (grade 1 product, made byKatayama Chemical, Inc.) in a filler-to-NMP weight ratio of 100:212.5 toform a slurry. The slurry was applied onto an aluminum/AlO_(x) sheet(30CB, made by Japan Capacitor Industrial Co., Ltd.; 250×150×0.030 mm)to a width of 90 mm, then dried (80° C.), rolled (packing density, about0.7 g/cm³) and cut to dimensions of 50.0 mm (width of coated area, 40.0mm)×20.0 mm to give electrodes.

Electrodes having a weight of about 0.092 g were selected as positiveelectrodes and electrodes having a weight of about 0.096 g were selectedas negative electrodes. Aluminum tape having a width of 3.0 mm waswelded to the positive electrode, and nickel tape having a width of 3.0mm was welded to the negative electrode.

An electrode group was formed by assembling, in opposition, two positiveelectrodes and two negative electrodes fabricated as described above,with two cellulose separators (FT40-35, made by Nippon KodoshiCorporation; thickness, 0.035 mm) cut to dimensions of 54.0×22.0 mmtherebetween. A sheet of the above-described 30CB (thickness, 30 μm;50.0 mm×20.0 mm) with aluminum tape welded thereto was also included inthe electrode group as an Al/AlO_(x) reference electrode, with anintervening separator.

The quaternary salt (ionic liquid) obtained in Synthesis Example 6 waspoured as the liquid electrolyte into the above electrode group in avolume equivalent to the volume of the above electrode group (100.0 vol%). The electrolyte-filled electrode group was then placed under avacuum of about 76 torr for 30 minutes and laminate-packed, giving anelectrical double-layer capacitor.

Comparative Example 3

A 1.0 M solution of tetraethylammonium-BF₄ in propylene carbonate(LIPASTE-P/EAFIN, produced by Toyama Chemical Co., Ltd.) as the liquidelectrolyte was poured into an electrode group obtained in the same wayas in Example 8 above to form an electrical double-layer capacitor.

The electrical double-layer capacitors obtained in above Example 8according to the invention and Comparative Example 3 were subjected tothe following electrical tests (1) to (3) to determine the initialcapacitance, the temperature dependence of the discharge properties, andthe self-discharge properties.

(1) Initial Capacitance

The following cycle was carried out three times. Charging at 10 mA and2.5 V to a current cutoff of 1 mA (25° C.), one hour of rest (25° C.),then discharging at 10 mA to a discharge cutoff of 0.0V (25° C.).

(2) Discharge Properties by Temperature

Charging was carried out at 10 mA and 2.5 V to a current cutoff of 1 mA(x° C.), followed by six hours of rest (x° C.), then discharging at 10mA to a discharge cutoff of 0.0 V (x° C.). The temperature values (x)were −20.0, 0.0, 25.0, 40.0 and 60.0.

(3) Self-Discharge Properties

Charging was carried out at 10 mA and 2.5 V to a current cutoff of 1 mA(25° C.), following which the capacitor was held at 60.0° C.

The results of the above electrical tests are shown in FIGS. 9 to 11.

It is apparent from FIG. 9 that the electrical double-layer capacitorobtained in Example 8 according to the invention achieves substantiallythe same amount of electricity as the capacitor obtained in thecomparative example.

FIG. 10 shows that the electrical double-layer capacitor obtained inExample 8 of the invention does not readily achieve a dischargecapacitance on the low-temperature side, but achieves a good dischargecapacitance on the high-temperature side.

FIG. 11 shows that the electrical double-layer capacitors obtained inExample 8 of the invention and Comparative Example 3 also havecomparable self-discharge properties.

It is thus apparent that even when an ionic liquid is used by itself asthe liquid electrolyte, an electrical double-layer capacitor having aperformance comparable to that obtained using a conventional organicelectrolyte solution can be obtained. Accordingly, from the standpointof cost and safety, such ionic liquids can be used by themselves asuseful liquid electrolytes for electrical storage devices.

Example 9

Secondary Battery

(1) Preparation of Electrolyte Solution

An electrolyte solution was prepared by mixing and dissolving 29.2 partsby weight of lithium bis(trifluoromethanesulfonyl)imide in 70.8 parts byweight of the quaternary salt (ionic liquid) obtained in SynthesisExample 6.

(2) Production of Positive Electrode

A paste-like positive electrode composition was prepared by stirringtogether and mixing the following: 91 parts by weight of LiCoO₂ as thepositive electrode active material, 3 parts by weight of Ketjenblack asthe conductive material, 60 parts by weight of a solution of 10 parts byweight of polyvinylidene fluoride (PVDF) dissolved in 90 parts by weightof N-methyl-2-pyrrolidone, and 15 parts by weight ofN-methyl-2-pyrrolidone.

The positive electrode composition was applied onto aluminum foil with adoctor blade to a film thickness when dry of 100 μm. This was followedby 2 hours of drying at 80° C., then rolling to give a LiCoO₂ positiveelectrode.

(3) Production of Lithium Secondary Battery

The positive electrode obtained as described above and metallic lithiumas the negative electrode were each cut to a diameter of 12 mm, apolyolefin porous membrane (E25MMS, made by Tonen Tapyrus Co., Ltd.) wasplaced as the separator between the above 12 mm diameter positive andnegative electrodes, and the electrolyte solution prepared as describedabove was poured and impregnated therein to form a coin-type lithiumsecondary cell.

Example 10

Secondary Battery

Aside from using an electrolyte solution prepared by mixing anddissolving 90.6 parts by weight of the quaternary salt (ionic liquid)obtained in Synthesis Example 1 and 9.4 parts by weight of lithiumtetrafluoroborate, a coin-type lithium secondary battery wasmanufactured in the same way as in Example 9.

The secondary batteries obtained in above Examples 9 and 10 weresubjected to a charge/discharge test in which the upper limit voltageduring charging was set at 4.2 V, the voltage cutoff during dischargewas set at 3 V, and the current density was 0.025 mA/cm². The test wascarried out by constant-current low-voltage charging andconstant-current discharging.

The discharge capacity, based on LiCoO₂ in the secondary battery inExample 9, was found to be 117.8 mAh/g, and the discharge capacity basedon LiCoO₂ in Example 10 was 115.4 mAh/g. Both are adequate values aslithium secondary batteries. FIG. 12 shows a graph of thecharge/discharge properties for the secondary battery obtained inExample 9.

As described above, because the electrolyte salt for electrical storagedevices according to the invention is a quaternary ammonium salt orquaternary phosphonium salt having at least one alkoxyalkyl group as asubstituent thereon, it has a low melting point and excellent solubilityin nonaqueous organic solvents. Thus, when liquid electrolytes forelectrical storage devices are prepared using these quaternary salts,the liquid electrolyte can be set to a higher concentration than in theprior art and the electrolyte salt does not deposit out at lowtemperatures. As a result, there can be obtained electrical storagedevices (e.g., secondary batteries and electrical double-layercapacitors, as well as other types of capacitors) which have excellentlow-temperature properties and have both a high charge/dischargecapacitance and a high electrostatic capacitance.

Because the ionic liquids of the invention are easy to manufacture andhandle, and have a broader potential window than organic ionic liquidsknown to the prior art, they lend themselves well to use as novelelectrolytes capable of functioning at temperatures below roomtemperature in the electrodeposition of metals and alloys, inelectroplating, and in electrochemical devices for storing energy, suchas various types of batteries and capacitors.

1. An electrical double-layer capacitor having: a pair of polarizableelectrodes, a separator between the polarizable electrodes and a liquidelectrolyte for electrical storage devices comprising an electrolytesalt having general formula (1) below,

 wherein R¹ to R⁴ are each independently an alkyl of 1 to 5 carbons oran alkoxyalkyl of the formula R′—O—(CH₂)_(n)—, R′ being methyl or ethyland the letter n being an integer from 1 to 4, and any two from amongR¹, R², R³ and R⁴ may together form a ring, with the proviso that atleast one of groups R¹ to R⁴ is an alkoxyalkyl of the above formula; Xis a nitrogen or phosphorus atom; and Y is a monovalent anion.
 2. Theelectrical double-layer capacitor of claim 1, wherein the electrolytesalt is a quaternary salt in which X is a nitrogen atom.
 3. Theelectrical double-layer capacitor of claim 2, wherein the electrolytesalt is a quaternary salt in which X is a nitrogen atom, R′ is methyland the letter n is
 2. 4. The electrical double-layer capacitor of claim1, wherein the electrolyte salt is a quaternary salt having generalformula (2) below

wherein R′ is methyl or ethyl, X is a nitrogen or phosphorus atom, Y isa monovalent anion, Me signifies methyl and Et signifies ethyl.
 5. Theelectrical double-layer capacitor of claim 1, wherein Y is BF₄ ⁻, PF₆ ⁻,(CF₃SO₂)₂N⁻, CF₃SO₃ ⁻ or CF₃CO₂ ⁻.
 6. The electrical double-layercapacitor of claim 4, wherein the quaternary salt has general formula(3) below

wherein Me signifies methyl and Et signifies ethyl.
 7. The electricaldouble-layer capacitor of claim 1, wherein the electrolyte salt has amelting point of up to 25° C.
 8. The electrical double-layer capacitorof claim 1, wherein the liquid electrolyte is composed solely of theelectrolyte salt of general formula (1).
 9. The electrical double-layercapacitor of claim 1, wherein the liquid electrolyte for electricalstorage devices comprises at least one electrolyte salt of generalformula (1) and a nonaqueous organic solvent.
 10. The electricaldouble-layer capacitor of claim 9, wherein the nonaqueous organicsolvent is a mixed solvent which includes as a main component ethylenecarbonate or propylene carbonate.
 11. The electrical double-layercapacitor of claim 9, wherein the nonaqueous organic solvent is oneselected from among ethylene carbonate, propylene carbonate, vinylenecarbonate, dimethyl carbonate, ethyl methyl carbonate and diethylcarbonate, or a mixed solvent of two or more thereof.
 12. The electricaldouble-layer capacitor of claim 1, wherein the polarizable electrodesinclude as a main component a carbonaceous material prepared from aresin.
 13. The electrical double-layer capacitor of claim 12, whereinthe resin is a phenolic resin or a polycarbodiimide resin.
 14. Theelectrical double-layer capacitor of claim 12, wherein the carbonaceousmaterial is prepared by carbonizing a phenolic resin or polycarbodiimideresin, then activating the carbonized resin.
 15. The electricaldouble-layer capacitor of claim 8, further comprising an ion-conductivesalt combined with the liquid electrolyte.
 16. The electricaldouble-layer capacitor of claim 15, wherein the ion-conductive salt is alithium salt.
 17. The electrical double-layer capacitor of claim 15,further comprising a nonaqueous organic solvent combined with the liquidelectrolyte.
 18. A secondary battery having: a positive electrode and anegative electrode, a separator between the positive and negativeelectrodes, and an electrolyte solution for electrical storage devices,wherein the electrolyte solution is composed of an ion-conductive saltwhich is solid at ambient temperature and a liquid electrolyte forelectrical storage devices which is characterized by being composedsolely of an ionic liquid characterized by having general formula (1)below,

wherein R¹ to R⁴ are each independently an alkyl of 1 to 5 carbons or analkoxyalkyl of the formula R′—O—(CH₂)_(n)—, R′ being methyl or ethyl andthe letter n being an integer from 1 to 4, and any two from among R¹,R², R³ and R⁴ may together form a ring, with the proviso that at leastone of groups R¹ to R⁴ is an alkoxyalkyl of the above formula; X is anitrogen or phosphorus atom; and Y is a monovalent anion.
 19. Anelectrical double-layer capacitor having: a pair of polarizableelectrodes, a separator between the polarizable electrodes and anelectrolyte solution for electrical storage devices, wherein theelectrolyte solution is composed of an ion-conductive salt which issolid at ambient temperature and a liquid electrolyte for electricalstorage devices which is characterized by being composed solely of anionic liquid characterized by having general formula (1) below,

wherein R¹ to R⁴ are each independently an alkyl of 1 to 5 carbons or analkoxyalkyl of the formula R′—O—(CH₂)_(n)—, R′ being methyl or ethyl andthe letter n being an integer from 1 to 4, and any two from among R¹,R², R³ and R⁴ may together form a ring, with the proviso that at leastone of groups R¹ to R⁴ is an alkoxyalkyl of the above formula; X is anitrogen or phosphorus atom; and Y is a monovalent anion.
 20. Anelectrical storage device having: a positive electrode composed ofactivated carbon, a negative electrode composed of a carbonaceousmaterial that is capable of occluding and releasing lithium ions, and anelectrolyte solution for electrical storage devices, wherein theelectrolyte solution is composed of an ion-conductive salt which issolid at ambient temperature and a liquid electrolyte for electricalstorage devices which is characterized by being composed solely of anionic liquid characterized by having general formula (1) below,

wherein R¹ to R⁴ are each independently an alkyl of 1 to 5 carbons or analkoxyalkyl of the formula R′—O—(CH₂)_(n)—, R′ being methyl or ethyl andthe letter n being an integer from 1 to 4, and any two from among R¹,R², R³ and R⁴ may together form a ring, with the proviso that at leastone of groups R¹ to R⁴ is an alkoxyalkyl of the above formula; X is anitrogen or phosphorus atom; and Y is a monovalent anion.